To safeguard human health it is important to prevent health-hazardous airborne particles from being inhaled. Particularly health-hazardous airborne particles are ultra fine particles, which are particles that have an equivalent diameter between about 10 nm and about 2.5 μm, more in particular between about 20 nm and about 300 nm. Ultra fine particles can be formed as a result of an incomplete combustion process, and they can be emitted into air from the exhaust of combustion sources such as automobile traffic and other local combustion sources. It is well-known that inhalation of ultra fine particles can result in severe lung injuries.
Local detection of airborne particles preferably involves determination of the total particle number concentration and the average diameter of airborne particles.
A device for determining the aforementioned parameters is known from WO 2007/000710 A2. The known device is arranged to sample an air flow and comprises a particle concentration variation section, capable of causing a variation of the concentration of ultra fine particles between at least a first concentration level and a second concentration level during at least one time interval. The particle concentration variation section is located upstream from a particle sensing section, capable of producing a measurement signal varying in dependence of the variation between the first concentration level and the second concentration level. In response to the applied particle concentration variation, the known device determines measurement signals associated with varied particle concentration levels in a serial way during successive time intervals. A set comprising at least two measurement signals corresponding with a set of at least two varied particle concentration levels is required for determining the total particle number concentration and the average particle diameter. Different sets of measurement signals can be determined periodically to follow the evolution of the total particle number concentration and the average particle diameter in the course of time.
For an accurate determination of the total particle number concentration and the average diameter of airborne particles, the known device requires an environment wherein the total concentration of airborne particles and the particle size distribution (i.e. the particle concentration as a function of particle size) should be no more than only a slowly-varying function of time, preferably substantially stationary in time. During the time interval required to measure a set of serial measurement signals required for a single determination of the total particle number concentration and the average particle diameter, the total particle number concentration and the average particle diameter should remain substantially constant. This time interval cannot be made arbitrarily small because of minimum required demands on the measurement accuracy that normally necessitate signal averaging during at least a minimum period of time. For accurate operation in a non-stationary environment (such as existing at or near a location where motorized traffic is present), a device is required that can determine the total particle number concentration and the average diameter of airborne particles also under highly transient conditions wherein these parameters may rapidly change during the course of time. Such circumstances can for instance arise at or near a location where motorized traffic is present.